Adhesive tapes with films based on polyvinyl chloride and with adhesive compositions based on solvent-containing natural rubber are known and are supplied by known manufacturers.
A description of such tapes is given, for example, in “Packaging Tapes” by Stefan Röber in Handbook of Pressure Sensitive Adhesive Technology, Third Edition, edited by Donatas Satas, Satas & Associates, Warwick, R.I. pp. 787–814. Further descriptions can be found in European Adhesives & Sealants 10(4), 1993, 29, by G. Pedala, and European Adhesives & Sealants 2(2), 1985, 18, by R. W. Andrew.
Owing to the known disadvantages associated with the production of solvent-containing adhesive tapes, such as, for example, the difficulty of recycling solvents, the emission of solvent to the environment, the workplace hazard due to highly flammable solvents, and the restriction on coating speed imposed by the drying of the adhesive composition, solvent-free technologies for producing adhesive tapes are advancing in importance.
A disadvantage to add to the above-described problems of the solvent technology is the need to break down the natural rubber during the production of the adhesive composition, in a process known as mastication. This results in the adhesive composition having a cohesiveness which decreases markedly in accordance with the extent of breakdown under mechanical load.
The deliberate industrial process of breaking down rubber under the combined action of shear stress, temperature, and atmospheric oxygen is referred to in the technical literature as mastication and is generally carried out in the presence of chemical auxiliaries, which are known from the technical literature as masticating agents or peptizers, or, more rarely, as “chemical plasticizing aids”.
In rubber technology, the mastication step is needed in order to make it easier to integrate the additives.
According to Römpp (Römpp Lexikon Chemie—Version 1.5, Stuttgart/New York: Georg Thieme Verlag 1998) mastication is a term used in rubber technology for the breaking down of long-chain rubber molecules in order to increase the plasticity and/or reduce the (Mooney) viscosity of rubbers. Mastication is accomplished by treating, in particular, natural rubber in compounders or between rolls at very low temperatures in the presence of masticating agents. The high mechanical forces which this entails lead to the rubber molecules being “torn apart”, with the formation of macro radicals, whose recombination is prevented by reaction with atmospheric oxygen. Masticating agents such as aromatic or heterocyclic mercaptans and/or their zinc salts or disufides promote the formation of primary radicals and so accelerate the mastication process. Activators such as metal (iron, copper, cobalt) salts of tetraazaporphyrins or phthalocyanines permit a reduction in the mastication temperature. In the mastication of natural rubber, masticating agents are used in amounts from about 0.1 to 0.5% by weight in the form of masterbatches, which facilitate uniform distribution of this small amount of chemicals in the rubber mass.
Mastication must be clearly distinguished from the breakdown known as degradation which occurs in all of the standard solvent-free polymer technologies such as compounding, conveying, and coating in the melt.
Degradation is a collective term for different processes which alter the appearance and properties of plastics. Degradation may be caused, for example, by chemical, thermal, oxidative, mechanical or biological influences or else by exposure to radiation (such as (UV) light). Consequences are, for example, oxidation, chain cleavage, depolymerization, crosslinking, and/or elimination of side groups of the polymers. The stability of polymers toward degradation may be increased by means of additives: for example, by adding stabilizers such as antioxidants or light stabilizers.
The use of adhesive compositions of this kind based on natural rubber hotmelt pressure-sensitive adhesives for adhesive tapes, especially packaging tapes, for sealing cartons made from recycled paper, or cardboard packaging, results in premature opening of the cartons. If there is sufficiently great tension across the lid of the carton, caused by the pressure of the packaged material in the carton or by the tension of the packaging material, which opposes sealing, the adhesive tape becomes detached from the surface of the carton and the carton opens as result of slippage of the packaging tape.
The cohesion and, with it, the packaging security afforded by adhesive tapes with an adhesive composition based on natural rubber may be improved either by crosslinking the rubber adhesive composition and/or by means of a variant preparation of the adhesive composition in the course of which the natural rubber used is broken down to a much-reduced extent and therefore has a higher molecular weight. This makes it possible to counter slippage of the adhesive tapes on the carton surface, as described above.
Adhesive tapes with a backing based on PVC, in combination with an adhesive based on acrylate dispersions, are not possible owing to the high polarity of the adhesive and the resultant extremely high unwind forces.
Solvent-free technologies for producing adhesive tapes, especially packaging tapes, have to date been restricted to the use of adhesive composition based on acrylate dispersions and to the use of melting, thermoplastic elastomers.
An advantage of these thermoplastic elastomers, predominantly block copolymers containing polystyrene blocks, is the relatively low softening point and the corresponding simplification of the application or coating process, and also the avoidance of the above- described disadvantages of the solvent-based technologies.
The unfavorable aging behavior and the profile of properties of such packaging tapes at elevated temperatures, the poor thermal stability, lead to premature opening of the cartons packaged with these tapes and/or a packaging tape performance profile which generally has some adverse features. Moreover, the process temperatures, which are very high in some cases, are detrimental to certain types of adhesive composition.
Further solvent-free systems based on aqueous adhesives, such as adhesive systems based on polyvinyl acetate, polyvinyl acetate-ethylene copolymers, neoprene, styrene-butadiene, polyurethane and polyvinyl alcohol, for example, are used little if at all owing to a costs and/or performance structure which is unfavorable for adhesive tapes, particularly packaging tapes.
An overview of the most important aqueous adhesive systems and also their use is given in “Solvent free adhesives”, T. E. Rolando (H. B. Fuller) in Rapra Rev. Rep. 1997, 9(5), 3–30 Rapra Technology Ltd.
Various routes to the solvent-free preparation and processing of pressure-sensitive rubber adhesives are known.
An overview of such adhesive compositions and their use in the field of pressure sensitive adhesives (PSAs) is given in “Natural Rubber Adhesives” (G. L. Butler in Handbook of Pressure Sensitive Adhesive Technology, Third Edition, edited by Donata Satas, Van Nostrand Reinhold New York, pp. 261–287).
All of the known processes are characterized by very extensive rubber breakdown. For the further processing of the compositions for self-adhesive tapes, this necessitates extreme crosslinking conditions and also has the consequence of an application profile which is to some extent restricted, especially as regards the use of resultant self-adhesive tapes at relatively high temperatures.
Solvent-free hotmelt PSAs based on nonthermoplastic elastomers, such as natural rubber or other high molecular weight rubbers, for example, lack sufficient cohesion for the majority of applications, in the absence of a step of crosslinking the adhesive composition, and are therefore unsuited to use in the context of an adhesive packaging tape. The reason for this failure of noncrosslinked adhesive compositions based on natural rubber is the relatively large reduction in molecular weight as a result of processing, and/or as a result of the preparation process of the adhesive compositions based on natural rubber, and the resultant reduced or inadequate cohesion of the adhesive compositions.
CA 698 518 describes a process for achieving production of a composition by adding high proportions of plasticizer and/or by simultaneously strong mastication of the rubber. Although this process can be used to obtain PSAs having an extremely high tack, the achievement of a user-compatible shear strength is possible only to a limited extent, even with a relatively high level of subsequent crosslinking, owing to the relatively high plasticizer content or else to the severe breakdown in molecular structure of the elastomer to a molecular weight average of Mw≦1 million.
The use of polymer blends, where besides nonthermoplastic natural rubber use is also made of block copolymers, in a ratio of approximately 1:1, represents essentially an unsatisfactory, compromise solution, since it results neither in high shear strengths when the self-adhesive tapes are used at relatively high temperatures nor in significant improvements on the properties described in CA 698 518.
Raw natural rubber latex is supplied from the plantations and is purified, preserved, and concentrated by means of appropriate methods. A general description of the types of latex and methods of processing is given in “Naturkautschuk—Technisches Informationsblatt” [Natural rubber—technical information sheet], Malaysian Rubber Producers Research Association, L1, 1977, and in “Kautschuktechnologie” [Rubber technology], Werner Hofmann, Gentner Verlag, Stuttgart p. 51 ff.
Natural rubber latex is harvested as a natural product from trees known as latex trees. After various processing steps for separation and purification, four basic types of natural rubber lattices are distinguished:                1. Standard natural rubber latex with a high ammonia content of 0.7% by weight        2. Natural rubber latex with a low ammonia content of 0.2% by weight in combination with zinc oxide and tetramethylthiuram disulfide<0.035% by weight        3. Natural rubber latex doubly centrifuged, of particularly high purity        4. Natural rubber latex partly vulcanized, for specialty applications        
Mastication and the associated breakdown of the natural rubber is not necessary when preparing adhesive compositions based on natural rubber latex, since in the case of the solvent-free aqueous adhesive composition the simple mixing of the components is not accompanied by mastication and thus by a breakdown of the molecule under mechanical load. This results in a close interlooping of the natural rubber latex/isoprene molecules, a high molecular weight owing to absence of mastication, a broad molecular weight distribution, and a low softening point Tg.
These factors result in a very good balance between cohesion and adhesion in the adhesive composition resulting from resin blends, and also in a profile of properties which is of very high performance for adhesive tapes, over a very wide temperature range.
These positive characteristics over a wide temperature range are exploited for the adhesive tape described in JP 56 030 481, comprising an adhesive composition based on natural rubber latex. Through the use of the natural rubber latex, a robustness to impact at low temperatures is described.
A further point is that natural rubber latex features a particularly wide spectrum of applications. Thus natural rubber latex is suitable on the one hand for use as an adhesive composition for packaging materials and on the other hand for applications outside of adhesive technology.
A disadvantage of adhesive composition systems based on natural rubber latex is the poor stability of the natural rubber latex raw material to mechanical load and the associated coagulation of the natural rubber latex in the adhesive composition. This sensitivity to shearing restricts the ease of handling and the usefulness of the natural rubber latex for adhesive composition technology and the adhesive tape industry. The production of adhesive tapes for which the adhesive composition comprising natural rubber latex can be coated at high speed onto a film is not known.
Through the use of stabilizing resin dispersions and/or surface-active substances, such as emulsifiers, for example, it is possible to increase significantly the stability of the natural rubber latex. However, increasing the stability of natural rubber latex using surface-active substances at high concentrations goes hand in hand with a drastic loss of cohesion of the adhesive composition, with the consequence that adhesive compositions modified in this way cannot be used for packaging tapes.
The general use of natural rubber latex for adhesive compositions and also latex-stabilizing resin dispersions is described in “Tackified waterborne adhesive for PSA tapes”, J. G. de Hullu, European Adhesives & Sealants, 12 (1998), p. 11–12. Through the use of such natural rubber latex-stabilizing resin dispersions, it becomes possible to process or coat adhesive compositions based on natural rubber latex. The trouble-free application of the dispersion adhesive compositions to a polymeric backing enables an industrially practicable and economically rational adhesive tape production process. The use of this technology is made possible by employing a suitable adhesive composition and by technically setting and optimizing the coating lines.
The application and development of an adhesive tape which unwinds quietly and is suitable for packaging applications has not been described in this context. The use of adhesive tapes with quiet unwind, particularly for carton sealing, however, is of great importance within the packaging industry in order to reduce the noise load in the packing houses and hence to increase work performance. The capacity for quiet unwinding is of fundamental importance in order to position an adhesive packaging tape within the middle to top price/performance segment.
In general terms, natural rubber latex is a known component of adhesive compositions and is described, inter alia, in patents. The fields of use of such adhesive compositions based on natural rubber latex are applications in the sector of labels and plaster technology.
What has not been disclosed, however, is the application of the adhesive to a continuous, PVC-based backing. Both properties are essential for the use of adhesives based on natural rubber latex.
The preparation of aqueous adhesive compositions based on natural rubber latex is known. These adhesive compositions are used as described above for producing self-adhesive labels. This is done using the customary methods of preparing dispersion adhesives.
The preparation of adhesive composition systems which comprise natural rubber latex is likewise described in EP 0 960 923 A1. This discloses not only the preparation of dispersion adhesive compositions but also the incorporation of natural rubber latex into other natural rubber adhesive systems using compounders, mixers or extruders. The adhesive compositions prepared in this way may be coated onto film or nonwoven. An improvement in the packaging properties of such adhesive tapes by means of a controlled, enduring crosslinking of the adhesive composition, which also may be used as part of a rational production process, has not been described.
When using adhesive composition variants based on natural rubber latex it is the case —as already set out above—that enduring crosslinking is unnecessary for achieving good packaging qualities that are superior to the majority of other adhesive composition systems.
Through the use of the above-described stabilizing resin dispersions and also by using natural rubber latex, and accordingly in the absence of any mechanical breakdown of the rubber whatsoever, it is possible to prepare, process, and apply adhesive compositions based on natural rubber latex and thus to produce adhesive tapes, since the adhesive composition possesses very high cohesion and hence optimum packaging security.
Not only for adhesive packaging tapes for sealing cartons but also for other adhesive packaging tapes, such as strapping tapes, therefore, the natural rubber-based adhesive composition must have sufficient cohesion.
Thermoplastic films based on polyvinyl chloride (PVC) are used to produce adhesive tapes by a variety of manufacturers. Particularly films based on PET are distinguished by high elongation at break and thermal stability of from 130° C. to 175° C., and resistance toward dilute alkalis and acids. Moreover, films based on polyesters possess a very high abrasion resistance and penetration resistance, but are less widespread in the field of adhesive packaging tapes owing to their relatively high price in relation to polyolefin-based films.
The PVC here may be obtained by emulsion, suspension or bulk polymerization. Copolymers based on PVC/vinyl acetate are also known.
Films based on PVC are supplied, in combination with solventborne adhesives based on natural rubber, as adhesive packaging tapes. In this case, owing to the better thermal stability, films of unplasticized PVC are used, which possess good thermal stability up to max. 105° C.
Films based on embossed PVC are likewise used in the adhesive tapes industry in combination with solventborne natural rubber compositions. By this means, a reduction in the unwind force and thus improved processing properties are achieved.
In the production of adhesive tapes with adhesive compositions based on natural rubber, both water-based and solventborne primers are used as adhesion promoters between adhesive composition and backing film. These adhesion promoters that are used possess in part a crosslinking effect on the natural-rubber-based adhesive composition which is applied from solution.
Furthermore, the films based on unplasticized PVC are distinguished by moderate penetration force and a partial resistance to petroleum spirit, oil, and alcohols. Generally speaking, the anchoring of primers to PVC films without corona treatment is better than in the case of other thermoplastic films, such as polyolefin-based films, for example. This derives from the high surface energy even without pretreatment [approximately 39 dyn/cm] and the rough surface texture of PVC films. For sufficient anchoring of adhesives based on natural rubber, a primer-coated PVC film is advisable.
It is an object of the present invention to provide adhesive tapes comprising films based on PVC and solventlessly prepared adhesive compositions based on natural rubber latex, said tapes exhibiting quiet unwind and so possessing outstanding suitability for use as packaging tapes for commercial cardboard packaging.